An organic semiconductor is expected to be applied to a photoelectric conversion element such as an organic thin film solar cell, an organic/inorganic hybrid solar cell, a light emitting element, and a photosensor. Especially, using a high molecular compound as an organic semiconductor material enables application of a low-cost coating method in fabrication of an active layer. In view of an energy demand and an emission reduction of CO2, a solar cell is expected as one of clean energies with low environmental burdens and its demand is rapidly increasing. A silicon-based solar cell is mainstream at present, but efficiency thereof is about 15%, and it is difficult to curtail a cost. A CdTe solar cell is also known as a solar cell which can be fabricated at a low cost. However, since Cd being a toxic element is used, there is a possibility that an environmental problem occurs. Under the circumstances, development of an organic thin film solar cell and an organic/inorganic hybrid solar cell as a next-generation solar cell which is low in cost, high in energy conversion efficiency, and nontoxic is expected.
In order to put an organic thin film solar cell to practical use, improvement of power generation efficiency of the organic thin film solar cell is intensely demanded. In order to improve the power generation efficiency, improvement of an open circuit voltage (Voc) is important. A value of the open circuit voltage of the organic thin film solar cell greatly depends on a combination of an electron donor and an electron acceptor, and materials used for them are required to be optimized. It is known that the open circuit voltage of the organic thin film solar cell is correlated with a difference between an energy level of a highest occupied molecular orbit (HOMO) of a p-type material and an energy level of a lowest unoccupied molecular orbit (LUMO) of an n-type material. It is thought that, in an organic thin film solar cell whose development is currently in progress, fullerenes such as phenylC61butyric acid methylester (PCBM) are most suitable as the n-type semiconductor material. As a generally used p-type semiconductor material, a conjugated polymer of polythiophene such as poly(3-hexylthiophene-2,5-diyl) (P3HT) can be cited.
The open circuit voltage (Voc) of the organic thin film solar cell using the combination of PCBM and P3HT is low, that is, about 0.6 V, and is not necessarily satisfactory in view of practical use. A possible method to improve a value of the open circuit voltage is to lower the HOMO level of the p-type semiconductor material. In this case, however, a band gap of the p-type semiconductor widens, which makes it impossible to absorb light in a long wavelength range. That is, absorption efficiency of light in a long wavelength side of a visible range decreases and thus entering light cannot be effectively used. As a result, there is a drawback that energy efficiency does not increase. The open circuit voltage value and the absorption of the light in the long wavelength range are often in a trade-off relation, and it is difficult to achieve the both at higher level.
As one attempt to improve the open circuit voltage value of the organic thin film solar cell, using, as the p-type semiconductor material, a polymer in which imide is ring-condensed to thiophene is being studied. In the organic thin film solar cell using the polymer in which imide is ring-condensed to thiophene as the p-type semiconductor, the open circuit voltage improves to about 0.85 V, but power generation efficiency is 1% or less, and is required to be further improved. The above circumstances have led to a demand for a p-type semiconductor material which not only increases an open circuit voltage value of an organic thin film solar cell but also improves an absorbing characteristic of light in a long wavelength range. Further, in an organic thin film solar cell, improvement of life in addition to improvement of the open circuit voltage is required. In order to improve the life, active substances (a donor and an acceptor) excellent in heat stability and so on are required.
Further, researches have recently been made on an organic/inorganic hybrid solar cell whose energy conversion efficiency is improved by using an organic/inorganic hybrid perovskite compound or an inorganic perovskite compound for a photoelectric conversion layer. In the organic/inorganic hybrid solar cell, polyarylamine or 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-bifluorene(spiro-OMeTAD) is used as a hole transport layer. Further, in order to enhance conversion efficiency, a dopant such as t-butylpyridine (TBP) or bis(trifluoromethanesulfonyl)imidelithium (Li-TFSI) is used for the hole transport layer. However, since TBP is liquid and Li-TFSI is a hygroscopic substance, there occur performance deterioration caused by diffusion or dissipation of TBP to the photoelectric conversion layer due to a temperature increase, or by absorption of water molecules due to deliquescence of Li-TFSI, and so on. This is a cause to shorten the life of the organic/inorganic hybrid solar cell. It has been also proposed to use P3HT being a p-type material as the hole transport layer, but sufficient power generation efficiency cannot be obtained in this case.